1. Field of the Invention
The present invention relates to a crystalline thin film and a process for production thereof, an element employing the crystalline thin film, a circuit employing the element, and a device employing the element or the circuit, which are useful for large-scale integrated circuits requiring high spatial uniformity such as flat panel displays, image sensors, magnetic recording devices, and information/signal processors.
2. Related Background Art
Flat panel displays such as liquid crystal displays have been improved in fineness, display speed, and gradation of image display by monolithic implementation of an image driving circuit to the panel. The simple matrix-driven panels have been replaced with active matrix-driven panels having a switching transistor for each pixel. At present, ultra-fine full-color liquid crystal displays are provided to be suitable for moving pictures by implementing a shift resistor circuit on the periphery of the same panel for driving the active matrix.
The monolithic implementation including the peripheral driving circuit can be produced at a practical production cost mainly owing to development of the technique for forming a polycrystal silicon thin film having excellent electrical performance on an inexpensive glass substrate: the technique in which amorphous silicon thin film deposited on a glass substrate is melted and resolidified by a short-time pulse projection of light of ultraviolet region such as an excimer laser by keeping the glass substrate at a low temperature. The crystal grain obtained by melting-resolidification has a low defect density in the grain in comparison with crystal grains obtained from the same amorphous silicon thin film by solid-phase crystallization into a polycrystalline thin film. Thereby, the thin film transistor constituted by using this thin film as the active region exhibits a high carrier mobility. Therefore, even with the polycrystalline thin film having an average grain size up to a submicron, an active matrix-driven monolithic circuit can be produced which exhibits sufficient performance in a liquid display having a fineness of 100 ppi or lower in diagonal display size of several inches.
However, it has become clear that the current thin film transistor employing the polycrystalline silicon thin film produced by melting-resolidification is still insufficient in performance for a liquid-crystal display of the next generation having a larger screen or a higher fineness. Furthermore, the aforementioned polycrystalline silicon thin film insufficiently performs as the driving circuit element in promising future application fields of plasma displays and electroluminescence displays driven at a higher voltage, or larger electric current than the liquid crystal display, or in the application fields of a medical large-screen X-ray image sensor. The polycrystalline silicon thin film, which has average grain size up to submicron, cannot give a high-performance element even with the low defect density in the grain, because of many grain boundaries which hinder charge transfer in the active region of the element having a size of about a micron.
For decreasing the grain boundary density, it is very effective to enlarge the average crystal grain size, which is inversely proportional to the grain boundary density. Also, in short-time melting-resolidification of the amorphous silicon thin film, several methods are disclosed to enlarge the average grain size up to a micron or larger. However, by the disclosed methods, while the average grain size is enlarged, the grain size distribution is broadened [e.g., J. S. Im, H. J. Kim, M. O. Thompson, Appl. Phys, Lett. 63, 1969 (1993); H. Kuriyama, T. Honda, S. Ishida, T. Kuwahara, S. Noguchi, S. Kiyama, S. Tsuda, and S. Nakano, Jpn. Appl. Phys. 32, 6190 (1993)]; and the spatial location of the grain boundaries is not controlled [e.g., T. Sameshima, Jpn. J. Appl. Phys. 32, L1485 (1993); H. J. Kim and J. S. Im, Appl. Phys. Lett. 68, 1513 (1996)]. Thereby, the increase of the average grain size will increase variation of the grain boundary density among the active regions of the element to result in an increase of variation of element performance. Otherwise, the method itself is extremely tricky to be not suitable for practical production [e.g., D. H. Choi, K. Shimizu, O. Sugiura, and M. Matsumura, Jpn. J. Appl. Phys. 31, 4545 (1992); and H. J. Song, and J. S. Im, Appl. Phys. Lett. 68, 3165 (1996)].
Regarding these problems, no idea is given for solving the latter problem, but a methodology has been developed for solving the former problem, in which the location of the grain boundaries and grain size distribution are controlled by controlling the location of formation of crystal grains. This is demonstrated in chemical vapor deposition of polycrystalline thin films and solid-phase crystallization of thin films (see e.g., H. Kumomi and T. Yonehara, Jpn. J. Appl. Phys. 36, 1383 (1997); and H. Kumomi and F. G. Shi, “Handbook of Thin Films Materials”, volume 1, chapter 6, “Fundamentals for the formation and structure control of thin films: Nucleation, Growth, Solid-State Transformations” edited by H. S. Nalwa (Academic Press, New York, 2001)).
Several attempts are reported to realize the above idea in formation of a crystalline thin film by melting-resolidification. Noguchi [Japanese Patent Application Laid-Open No. 5-102035], and Ikeda [T. Noguchi and Y. Ikeda: Proc. Sony Research Forum, 200 (Sony Corp., Tokyo, 1993)], in melting-resolidification of amorphous silicon thin film by excimer-laser annealing, prepare openings through a light-intercepting layer provided on an amorphous silicon thin film and project an excimer laser beam to the amorphous silicon thin film. Thereby, the amorphous silicon at the opening portions is selectively melted and resolidified to form preferential crystallization region in the thin film plane. Toet et al. [D. Toet, P. V. Santos, D. Eitel, and M. Heintze, J. Non-Cryst. Solids 198/200, 887 (1996); D. Toet, B. Koopmans, P. V. Santos, R. B. Bergmann, and B. Richards, Appl. Phys. Lett. 69, 3719 (1996); and D. Toet, B. Koopmans, R. B. Bergmann, B. Richard, P. V. Santos, M. Arbrecht, and J. Krinke, Thin Solid Films 296, 49 (1997)] project an Ar laser beam focused to about 1 μm onto an amorphous silicon thin film to cause melting and resolidification only in the irradiated regions.
However, in the above methods, many crystal grains are formed in the crystallization regions at random positions of the grains in the region, and the number of crystal grains formed in the regions varied greatly among the regions.
Ishihara and Wilt [R. Ishihara and P. Ch. van der Wilt, J. Appl. Phys. 37, L15 (1998); and P. Ch. van der Wilt and R. Ishihara, Phys. Stat Sol. (A)166, 619 (1998)] report a method of melting-resolidification of an amorphous silicon thin film by excimer-laser annealing on a single-crystalline silicon substrate covered with an oxide layer, in which method an oxide film is deposited on a single crystalline silicon substrate having hillocks and the deposited oxide film is flattened. Thereby, the underlying oxide layer is made thinner locally on the top of the hillocks and heat is transferred more rapidly from the thinner portion of the molten silicon film to the substrate to cause rapid cooling. Therefore, the solidification is initiated preferentially from the hillock portions to cause growth of the crystal grain. Ishihara et al. mention the possibility of growing a single crystal grain on the hillocks by optimizing the projected area of the hillocks and the oxide film thickness. However, the possibility has not been confirmed, and the window for the optimum conditions is very small. Moreover this method employs a highly heat-conductive underlying substrate, and is not applicable to a method employing a glass substrate. Furthermore, the fine working of the underlying substrate and the flattening of the oxide film over a large area is not easy and is not practicable.
As described above, in the formation of crystalline thin film by melting-resolidification, the location of the crystal grain formation cannot readily be controlled. However, this method is promising for improving the properties of the thin film, since the thin crystalline film formed by the melting-resolification method has defects in a low density in the crystal grain in comparison with the films prepared by other thin film formation methods.
The present invention intends to provide a process for producing a general-purpose crystalline thin film applicable to glass substrate or the like by melting-resolidification, in which the location of the crystal grains can precisely be controlled. The present invention intends to provide also a thin film in which the location of the crystal grains can be precisely controlled. The present invention intends further to provide a high-performance element and circuit, and to provide a device by employing the thin film.